1. Field of the Invention
The invention relates to oligonucleotide-quencher-fluorescent-dye conjugates having improved characteristics, and to reagents suitable for incorporating novel quencher and fluorescent dye moieties into oligonucleotides. The invention also relates to the use of oligonucleotide-quencher-fluorescent-dye conjugates in detection methods for nucleic acid targets.
2. Description of the Related Art
Synthetic oligonucleotides have been used for years as sequence specific probes for complementary DNA and RNA targets. These methods have broad application in forensics, molecular biology and medical diagnostics since they allow the identification and quantitation of specific nucleic acid targets. Early uses of DNA probes relied on radioactivity (typically 32P) as the label, while recent methods use reporter molecules which include chemiluminescent and fluorescent groups. Improved instrumentation has allowed the sensitivity of these spectroscopic methods to approach or surpass the radiolabeled methods. Recently developed detection methods employ the process of fluorescence resonance energy transfer (FRET) for the detection of probe hybridization rather than direct detection of fluorescence intensity. In this type of assay, FRET occurs between a donor fluorophore (reporter) and an acceptor molecule (quencher) when the absorption spectrum of the quencher molecule overlaps with the emission spectrum of the donor fluorophore and the two molecules are in close proximity. The excited-state energy of the donor fluorophore is transferred to the neighboring acceptor by a resonance dipole-induced dipole interaction, which results in quenching of the donor fluorescence. If the acceptor molecule is a fluorophore, its fluorescence may sometimes be increased. The efficiency of the energy transfer between the donor and acceptor molecules is highly dependent on distance between the molecules. Equations describing this relationship are known. The Forster distance (Ro) is described as the distance between the donor and acceptor molecules where the energy transfer is 50% efficient. Other mechanisms of fluorescence quenching are also known, such as, collisional and charge transfer quenching.
Typically detection methods based on FRET are designed in such a way that the donor fluorophore and acceptor molecules are in close proximity so that quenching of the donor fluorescence is efficient. During the assay, the donor and acceptor molecules are separated such that fluorescence occurs. FRET-based detection assays have been developed in the fields of nucleic acid hybridization and enzymology. Several forms of the FRET hybridization assays are reviewed (Nonisotopic DNA Probe Techniques, Academic Press, Inc., San Diego 1992, pp. 311-352). Quenching can also occur through non-FRET mechanisms, such as collisional quenching (see, Wei et al., Anal. Chem. 66:1500-1506 (1994)).
Since its discovery, the polymerase chain reaction (PCR) has revolutionized molecular biology. This technique allows amplification of specific DNA sequences, thus allowing DNA probe assays to be executed from a single DNA target copy. PCR-based diagnostic assays have initially not been used routinely, in part due to problems with sample handling and possible contamination with non-source DNA. Recently, new homogeneous fluorescent-based DNA assays have been described which can detect the progress of PCR as it occurs (“real-time” PCR detection) using spectrofluorometric temperature cyclers. Two popular assay formats use DNA probes which become fluorescent as DNA amplification occurs (fluorogenic probes).
The first format for “real-time” PCR uses DNA probes known as “molecular beacons” (Tyagi et al., Nat. Biotech., 16: 49-53 (1998)). Molecular beacons have a hairpin structure wherein the quencher dye and reporter dye are in intimate contact with each other at the end of the stem of the hairpin. Upon hybridization with a complementary sequence, the loop of the hairpin structure becomes double stranded and forces the quencher and reporter dye apart, thus generating a fluorescent signal. Tyagi et al. reported use of the non-fluorescent quencher dyes including the dabcyl(4-{[4-(dimethylamino)phenyl]diazenyl}benzoyl moiety, absorbance max=453 nm) used in combination with fluorescent reporter dyes of widely varying emission wavelength (475-615 nm). At the time this was surprising since FRET requires significant overlap of the absorption spectrum of the quencher and of the emission spectrum of the reporter. In case of a dabcyl moiety containing (hereinafter “dabcyl”) quencher and some fluorescent dyes, the spectral overlap was extremely low, yet quenching efficiency was high. Therefore it was proposed that the mechanism of quenching for the hairpin form of the beacons was not FRET, but collisional quenching. In fact, the UV spectra of the quencher changes in the hairpin form of the beacon, providing evidence of the molecular contact and thus of collisional quenching. A related detection method uses hairpin primers as the fluorogenic probe (Nazarenko et al., Nucl. Acid Res. 25:2516-2521 (1997)).
The second format for “real-time” PCR uses DNA probes which are referred to as “5′-nuclease probes” (Lee et al., Nucl. Acid Res., 21: 3761-3766 (1993)). These fluorogenic probes are typically prepared with the quencher at the 3′ terminus of a single DNA strand and the fluorophore at the 5′ terminus. During each PCR cycle, the 5′-nuclease activity of Taq DNA polymerase cleaves the DNA strand, thereby separating the fluorophore from the quencher and releasing the fluorescent signal. The 5′-nuclease assay requires that the probe be hybridized to the template strand during the primer extension step (60-65° C.). They also disclose the simultaneous “real-time” detection of more than one polynucleotide sequence in the same assay, using more than one fluorophore/quencher pair. The 5′-nuclease PCR assay is depicted in FIG. 1.
Initially it was believed that 5′-nuclease probes had to be prepared with the quencher (usually tetramethylrhodamine (TAMRA)) positioned at an internal nucleotide in close proximity to the 5′-fluorophore (usually fluorescein (FAM) or tetrachlorofluorescein (TET)) to get efficient FRET. Later it was found that this is not necessary, and the quencher and the fluorophore can be located at the 3′ and 5′ end of the ODN, respectively. It has been proposed that the random coil structures formed by these fluorogenic probes in solution allow a 3′-quencher dye to pass within the Forster radius of the 5′-fluorophore during the excited state of the molecule.
A number of donor/acceptor pairs have previously been described, important to the present invention is dabcyl that is used for instance as a quencher of dansyl sulphonamide in chemosensors (Rothman & Still (1999) Med. Chem. Lett. 22:509-512).
Surprisingly, there have been no published reports on the use of dabcyl in 5′-nuclease probes or other FRET probes that use long wavelength fluorophores. As mentioned above, dabcyl was used in the beacon-type probes but this is a different quenching mechanism wherein the dabcyl and fluorophore are in intimate contact (collisional quenching). Dabcyl was used in fluorogenic peptides as a quencher for the fluorophore EDANS (5-[(2-aminoethyl)amino]naphthalene-1-sulfonic acid) which emits at short (490 nm, blue) wavelength (Matayoshi et al. Science 247: 954-958 (1990)). EDANS also has a lower extinction coefficient than dabcyl so it is not surprising that fluorescent quenching was efficient. It was found for the first time in the present invention that dabcyl can be used to quench fluorescein in a FRET type mechanism.
In addition to the 5′-nuclease PCR assay, other formats have been developed that use the FRET mechanism. For example, single-stranded signal primers have been modified by linkage to two dyes to form a donor/acceptor dye pair in such a way that fluorescence of the first dye is quenched by the second dye. This signal primer contains a restriction site (U.S. Pat. No. 5,846,726) that allows the appropriate restriction enzyme to nick the primer when hybridized to a target. This cleavage separates the two dyes and a change in fluorescence is observed due to a decrease in quenching. Non-nucleotide linking reagents to couple oligonucleotides to ligands have also been described (U.S. Pat. No. 5,696,251).
FRET systems also have applications in enzymology. Protease cleavable substrates have been developed where donor/acceptor dye pairs are designed into the substrate. Enzymatic cleavage of the substrate separates the donor/acceptor pair and a change in fluorescence is observed due to a decrease in quenching. Cleavable donor/acceptor substrates have been developed for chymotrypsin (Li et al. Bioconj. Chem., 10: 241-245 (1999)), aminopeptidase P (Hawthome et al., Anal. Biochem., 253: 13-17 (1997)), stromelysin (Bickett et al., Ann. N.Y. Acad. Sci., 732: 351-355 (1994)) and leukotriene D4 hydrolase (White et al., Anal. Biochem., 268: 245-251 (1999)). A chemosensor was described where binding of the ligand separates the donor/acceptor pair (Rothman et al., Biorg. Med. Chem. Lett., 9:509-512 (1999)).
U.S. Pat. No. 5,801,155 discloses that oligonucleotides (ODNs) having a covalently attached minor groove binder (MGB) are more sequence specific for their complementary targets than unmodified oligonucleotides. In addition the MGB-ODNs show substantial increase in hybrid stability with complementary DNA target strands when compared to unmodified oligonucleotides, allowing hybridization with shorter oligonucleotides.
Reagents for fluorescent labeling of oligonucleotides are critical for efficient application of the FRET assays described above. Other applications such as DNA micro arrays also use fluorescently labeled DNA probes or primers, and there is a need for improved reagents which facilitate synthesis of fluorescent DNA. In general, phosphoramidite reagents and solid supports are widely used on ODN synthesis. However, there are few commercially available phosphoramidite reagents for introducing fluorescent groups into ODNs.
Linker groups to attach different ligand groups to ODNs play an important role in the synthesis of oligonucleotide conjugates. A method for the synthesis of 3′-aminohexyl-tailed oligonucleotides (Petrie et al., Bioconj. Chem., 3:85-87 (1992)), the use of a trifunctional trans-4-hydroxy-L-prolinol group (Reed et al., Bioconjug. Chem., 2:217-225 (1991)), diglycolic acid (Pon et al., Nucl. Acids. Res., 25:3629-3635 (1997)), 1,3-diol reagents (U.S. Pat. Nos. 5,942,610 and 5,451,463) and a non-nucleotide trifunctional reagent (U.S. Pat. No. 5,696,251) have been reported.
Resorufin and coumarin derivatives have been extensively used as enzyme substrates to differentiate isozymes of cytochrome P450 (Haugland et al., Handbook of Fluorescent Probes and Research Chemicals, Six Edition, Eugene, Oreg., of the Several Views, pp. 235-236. 1996.). Reactive resorufin analogs have been disclosed in U.S. Pat. No. 5,304,645. Activated esters of coumarin derivatives are also known in the art (Hirshberg et al., Biochem., 37:10391-5 (1998)). Coumarin-labeled dUTP incorporated in probes were used for in situ hybridizations (Wiegant et al., Cytogenet. Cell Genet., 63:73-76 (1993)). Phosphoramidites to introduce labels into oligonucleotides have been described in U.S. Pat. Nos. 5,328,824 and 5,824,796.
Many current hybridization applications, require more than one reporter molecule. In addition although reporter fluorophores are available to be used in reporter/quencher pairs, most suffer from having some undesirable characteristics, e.g., mixtures are difficult to separate, they are positively charged or difficult to synthesize, unstable during oligonucleotide synthesis or having overlapping emission wavelengths with other desirable reporters.
The present invention provides reagents for oligonucleotide probes that address these unfavorable characteristics and overcome some or all of the difficulties.